1. Field of the Invention
This invention relates to radioactive isotope tracers and methods for their use and manufacture. In one aspect, sintered ceramic particles containing an element having the capability to be made radioactive are provided. In another aspect, radioactive sintered ceramic particles are provided. In still another aspect, a method of manufacturing radioactive particles is provided. In still another aspect, a method of using radioactive particles to locate particles in a wellbore or other piping system with negligible contamination of the system with radioactivity is disclosed.
2. Description of Related Art
Radioactive elements are commonly used for tracing the flow of liquids and solids in flow streams. The elements can be present as a soluble compound in a liquid, as insoluble or slightly soluble particles of the element or a compound of the element suspended in the flow stream, or as a soluble or insoluble compound attached to particles of other material which are suspended in a liquid or gas.
Slurries of particles are pumped into wells drilled through subterranean formations for several reasons. One reason is in connection with hydraulic fracturing of wells. The particles are called "proppant," and such particles function to fill the fracture created in the earth around a well and thereby to allow greater fluid flow rate into or out of the well. It is desirable to know after a well has been fractured the vertical extent of the proppant particles that have been placed around the well--particularly, whether the proppant is located in a zone of the well containing hydrocarbons or whether the proppant has been transported to another zone above or below the hydrocarbon-containing zone. It is common for radioactive particles to be added to the proppant as it is injected into the well. After the fracturing operation is complete, a logging tool is run into the well and the location of the proppant-radioactive particle mixture is located.
Slurries are also used in a well for cementing the casing in the well. The cement slurry is pumped into the well through the casing and flows upward outside the casing. It is important to know where the cement is located outside the casing in the well after it sets. Radioactive tracers are sometimes added to the cement slurry as it is pumped into a well. After the cement has set, a logging tool which measures gamma ray radiation is run into the well and the level of radiation is measured at different depths in the casing of the well. Different radioisotopes may be added to different portions of the cement slurry and the logging tool may be used to measure the location of the different tracers outside the casing. A spectral log may be used to indicate the distance of the tracer from the wellbore.
Another use of slurries in wells is in the process of gravel packing of wells, In this process particles, called "gravel," are placed near the wellbore and in the wellbore outside a screen to prevent formation solids from entering the wellbore or plugging the screen. It is important to know whether the gravel pack is continuous and how far it extends vertically in the well. Radioactive particles are added to the gravel as it is pumped into the well and a logging tool is run into the well after the gravel packing operations are complete to determine the location of the gravel.
Radioactive tracers are used in many other flow systems for measuring flow rates, flow patterns and other phenomena associated with movement of fluids or solids in industry or science. In many of these applications the radioactive tracers are placed directly in a liquid. There is often difficulty from the radioactive material plating on to surfaces or being disseminated through the flow system to contaminate the system with radioactivity.
U.S. Pat. No. 3,492,147 discloses a process for production of resin-coated solids, the resin coating incorporating radioactive materials. U.S. Pat. No. 4,731,531 discloses the use of particulate material which is nonradioactive until it is irradiated by neutrons at the surface of a well immediately before it is injected into the well or after it has been deposited in the formation around the well. The nonradioactive isotope is contained in an infusible resin coated on the surface of the particles. Radioactive particles having an infusible resin on the surface were sold by Halliburton Company under the trademark RAYFRAC.RTM.. Other radioactive particles sold for use in the oil industry are believed to be manufactured by simply immersing sand particles in a radioactive solution and drying the particles, the radioactivity then being trapped within natural cracks existing in the sand particles.
Techniques for detecting and measuring radioactivity are well known. A device such as a Geiger Counter will measure total radioactivity. Techniques for measuring the amount of radiation as a function of the energy of the gamma ray are also well known. Each radioactive isotope emits a characteristic spectrum of energies of radiation. Spectral analysis of the gamma rays from a radioactive isotope of an element used in the laboratory and surface facilities is well-known. In recent years, tools have been developed and made available for measuring the spectral analysis of gamma rays in wells. Spectral analysis makes possible use of multiple radioactive tracers in a flow system or well at the same time. In addition, technology has been developed to determine the relative distance from the detecting tool of different tracers, based on the phenomenon of Compton scattering of the gamma rays. One system for use in wells is sold by Halliburton Logging Services, Inc. under the trademark TRACERSCAN. This same detection technology could be used in other flow systems. The spectral log in a well makes possible both the vertical and radial distribution of tracers used in evaluating the effectiveness of hydraulic fracturing, cementing, and gravel packing operations. The article "Tracer Technology Finds Expanding Applications," Petroleum Engineer International, June, 1989, pp. 31-36, and references cited therein describe the new spectral analysis technology and its application to wells.
In the application of radiotracers in wells, preferably no tracers are left inside the casing, since only tagged material outside the casing contributes useful information regarding material placement within the formation. A severe limitation in using prior art radioactive particles which are initially radioactive or which are made radioactive by neutron bombardment before injection into a well or piping system is that radioactive material washes off particles or is abraded or is broken from the surface of the particles as they are pumped in a flow stream. This loss of radioactivity from the particles creates a background radiation at certain locations or throughout the well or piping system. The extraneous source of radiation can be a severe limitation in subsequent radioactive logging of wells and greatly diminishes the accuracy of measurements intended to be indicative of conditions outside the wellbore. In piping systems on the surface of the earth, radioactive contamination can be hazardous and can interfere with other operations.
Therefore, there is a great need for particles that can be made radioactive and particles that are radioactive which can be pumped into wells or other flow streams without loss of radioactivity and contamination of the flow stream. Further, a method of manufacturing such particles which allows incorporation of a variety of elements which can produce distinctive radioactive spectra is needed, and a method of employing these particles to locate slurries which have been injected into wells or other piping systems is needed.